Part9: Baudot and ASCII Codes

Introduction
This section introduces the concept of data codes. This refers to the way in which bits are grouped together to represent different symbols. There are a number of different codes, but the most common code in use today is the ASCII code.

DATA CODES
This refers to the way in which data is represented. The sender and receiver must use the same code in order to communicate properly. Here, we will briefly look at two common codes, one which was developed earlier on and was widely used in early telegraph systems, and the other, which is in widespread use today.

The Baudot Code
The Baudot code was used extensively in telegraph systems. It is a five bit code invented by the Frenchman Emile Baudot in 1870. Using five bits allowed 32 different characters. To accommodate all the letters of the alphabet and numerals, two of the 32 combinations were used to select alternate character sets.

Each character is preceded by a start bit, and followed by a stop bit. It is an asynchronous code, and thus suited for low speed data communication.

Value	LTRS shift	FIGS shift	Value	LTRS shift	FIGS shift
3	A	-	23	Q	1
25	B	?	10	R	4
14	C	:	5	S	.
9	D	Who are u	16	T	5
1	E	3	7	U	7
13	F	!	30	V	;
26	G	&	19	W	2
20	H	#	29	X	/
6	I	8	21	Y	6
11	J	Bell	17	Z	"
15	K	(	0	BLANK	BLANK
18	L	)	31	LTRS	LTRS
28	M	.	27	FIGS	FIGS
12	N	,	4	SPACE	SPACE
24	O	9	8	CR	CR
22	P	0	2	LF	LF

For instance, lets consider coding the phase "JAMES BOND 007 SAYS HI!" using the Baudot code. To switch between the LTRs and FIGs requires the use of a LetterShift or a FigureShift. Once switched, you stay in that mode till you want to switch back again. So, here is the phrase encoded in Baudot.

J	A	M	E	S		B	O	N	D		0	0	7		S	A	Y	S		H	I	!
31 11	3	28	1	5	4	25	24	12	9	4	27 22	22	7	4	31 5	3	21	5	4	20	6	27 13

Something to think about
Looking at the Baudot code, can you think of a reason why it is not suitable for widespread use today? [Answer]

ASCII (American Standard Code for Information Interchange)
The ASCII code is the most popular code for serial data communications today. It is a seven bit code (128 combinations), and thus supports upper and lowercase characters, numeric digits, punctuation symbols, and special codes. The table below lists the values for each character in the ASCII set.

NUL

SOH

STX

ETX

EOT

ENQ

ACK

BEL

TAB

DLE

DC1

DC2

DC3

DC4

NAK

SYN

ETB

CAN

SUB

ESC

(

)

;

[

]

{

}

DEL

To work out a particular value from the table, you first determine the row value, then add the column value. For example, the character A has a value of 41, being a row value of 40 and a column value of 1.

ASCII is also used as the data code for keyboards in computers. Control Codes have values between 00 and 1F (hexadecimal). Control codes are used in binary synchronous communication, and device control codes in communicating with devices such as printers or terminals.

A control code can be generated from a keyboard by holding down the Ctrl key and pressing another key. For instance, holding down the Ctrl key and pressing the A key generates the control code SOH.

Something to think about
Looking at the ASCII code, can you think of any limitation it might have for widespread use today? [Answer]

Summary
Data codes have always been in widespread use from mankinds early history. From the use of hand signals to mirrors flashing signals across the land, to smoke signals of the American Indians, information has been coded and sent by a variety of means.

ASCII is the most widespread data code in use today. It is a seven bit code, but with the world rapidly shrinking and global boundaries becoming blurred, the necessity to communicate across language barriers has exposed the limitations of this code. Another code, unicode, offers some promise in this multi-language area.